The battle against crime and terrorism has necessarily been a high priority for all civilizations. Since Sep. 11, 2001, homeland security has been receiving the highest attention in airports, post offices and other sectors dealing with public access. In the USA, the Department of Homeland Security (DHS) has recently been formed in addition to the FBI and CIA, indicating the priority of the U.S. government to prevent further attacks. In the UK, there have been frequent threats of terrorist attacks on airports.
While x-ray machines are commonly used to check airline passengers' luggage and they are effective in identifying objects of different densities, they have significant problems: (a) there are concerns over radiation exposure. In particular, the members of security staff working on x-ray scanners possibly receive unacceptable radiation; (b) the machines are expensive to purchase and operate. It is estimated that the operational cost is around $1 million a year per x-ray machine, prohibiting their use in less cash-intensive situations; (c) most x-ray machines can only provide 2D images, making it difficult for members of security staff to identify dangerous objects. The images generated must be viewed or assessed by experienced/trained personnel, because no efficient automatic methods are currently available for reliably triggering alarms, and (d) they are bulky and non-portable and security personnel cannot easily check an abandoned bag using an x-ray machine. Therefore, alternative and/or complementary security scanning tools are being sought.
For general aviation, e.g., small airports and non-standard operations, and for sports events or public theaters, it is not realistic to use x-ray machines routinely. The most popular tool used for security checking is the metal detector. Although metal detectors are very sensitive to ferrite materials, they cannot give information on shape and/or size and cannot detect non-metal objects (e.g., a ceramic knife) and chemical substances (e.g., explosives). These limitations of the currently-available detectors prevent the identification of dangerous objects and materials. In many other cases, it is also necessary to check bags and people for security reasons, e.g., to control access to train stations, government buildings, and even buses.
Neither post offices nor parcel delivery services currently have any tool for checking the contents of envelopes, packages and boxes. Bombs, anthrax, guns and other illegal materials have been sent through post offices and delivery services. This indicates the necessity of developing technology and security scanning tools to address this problem. To detect explosives and chemical substances, various chemical sensors have been developed, such as the Ionscan machine from Smith Detection (Elliot and Goetz 2003). These tools are designed to check for vapours from biological and dangerous chemical materials. However, if the explosive or chemical substances are well sealed, the chemical sensors will not detect them.
In the USA, InVision Technologies, Inc. has developed CT scanner machines for security applications, based on x-ray sensors (InVision 2003). The sensing principle of such machines is the same as the commonly-used x-ray machines, but can provide tomographic 2D and 3D images. However, those machines possess all the disadvantages of the commonly-used x-ray machines: radiation, expense and bulk. X-ray tomography sensors have to take measurements over extended periods of time, so that a usable set of data can be recorded to produce an acceptable image. Recently, InVision announced a new machine, QScan QR 500, which is based on quadrupole resonance analysis. In principle, this is similar to magnetic resonance imaging (MRI). Instead of using a magnet, quadrupole resonance analysis makes use of low intensity pulses of carefully tuned radio waves to probe for the molecular structure of target materials (InVision 2003). However, this type of machine is limited to the detection of some liquid explosive materials only, and is huge in size and massive in weight, a few tons.
The use of penetrating non-ionizing radiation for security checking, such as millimeter-waves, has also been proposed (QinetiQ 2001). Although the organizations involved assert that this is non-harmful radiation, it is well established that any millimeter-wave or microwave radiation is potentially harmful because the operating frequency is as high as 35 or 94 GHz (QinetiQ 2001). At the Pacific Northwest National Lab in the USA, similar scanners using millimeter-wave radiation have also been announced. They declared that the scanner allows security personnel at airports to “see” the full spectrum of concealed weapons, including non-metallic threats, such as plastics, explosives and ceramic knifes. Legitimate safety concerns, which have not been adequately addressed, would allow individuals to refuse to be “microwaved”. This would prevent security agencies from insisting on scanning everyone.
Tomography has been widely used in hospitals for diagnosis purposes. An x-ray CT scanner can provide 2D and 3D images of objects inside a human body. In recent years, industrial process tomography (IPT) has developed rapidly. While the principles of IPT are similar to medical tomography, there are some key differences: (a) IPT systems may be optimized to sense properties other than just density, e.g., contrast in dielectric properties, thus distinguishing different plastics; (b) IPT systems may acquire only a few dozen measurements per image. Whilst this yields images of relatively poor spatial resolution, the temporal resolution is very good, in the order of milliseconds; and (c) IPT systems are small, portable and relatively low cost.
Among IPT techniques, electrical tomography, which measures permittivity, conductivity and permeability and accordingly are called electrical capacitance tomography (ECT), electrical resistance tomography (ERT) and electro-magnetic tomography (EMT), offers several advantages: rapid response, low-cost, non-intrusive and/or non-invasive, and robustness in hostile environments. Electrical tomography has been used for many research applications, such as imaging of gas-oil two-phase flows in pipelines, gas-solids distribution in pneumatic conveyors and fluidised beds, combustion flames, liquid—liquid and solids-liquid mixing processes and personnel landmines (York 2001). The main weakness of electrical tomography is that it uses a relatively small number of sensing elements (typically 8, 12 or 16), because of the limitation in the sensitivity of electronic circuits. As a result, the number of independent measurements is usually limited to say <100, and, hence, the present electrical tomography systems can only provide moderate spatial image resolution.
Although tomography has been utilized in industrial and medical applications for producing images of the interiors of opaque objects over the past 35 years, characteristics of current tomography technology have limited its utility. The problems inherent in the sensors themselves include: (1) excessive size and weight of sensor devices using radiation, (2) safety of living tissue in scanned objects, and (3) low resolution of sensor signals. Other problems associated with the process, by which the sensor signals are converted into useful information, include: (1) integration of control of motion of object with sensor signal capture cycles, (2) cost of the entire apparatus, (3) lengthy time required between generation of sensor signal and creation of useful image, and (4) inability to distinguish among different types of materials.
There exists a need for new and improved security scanning devices for non-invasive checking of the contents of containers, and particularly luggage at transportation, freight and mail facilities.